Controls for boiler installations



March 1934- H. c. MITTENDORF r AL 1,951,763

CONTROLS FOR BOILER INTALLATIONS Filed June 27, 1930 4 Sheets-Sheet 1 aINVENTORS We? yam/u r masrfi March 1934- H. c. MITTENDORF El AL1,951,763

CONTROLS FOR BOILER INSTALLATIONS Filed June 27, 1930- 4 Sheets-Sheet 2INVEN OR a 4 5 Waxm er ATTORNEYJ March 20,1934. H. c. MITTENDORF ET-AL1,951,763

CONTROLS FOR BOILER INSTALLATIONS Filed June 27, 1930 4 Sheets-Sheet 3INVENTOR MLLW ATTORNEYg March 20, 1934. H. c. MITTENDORF ET AL ,7

CONTROLS FOR BOILER INSTALLATIONS Filed June 27, 1930 4 Sheets-Sheet 4mum,

I WMmjer/Y/ofor mmm' INVENTOR to the storage of coal in the milloccurring at Patented Mar. 20, 1934 UNITED STATES PATENT OFFICE CONTROLSFOR BOILER INSTALLATIONS Application June 27, 1930, Serial No. 464,132

11 Claims. (01. 110-400) such time, the mill becomes partlyplugged andalthough the coal feed rate may be quite low, even this amount of coalfeed tends to aggravate the plugging condition. The plugging actionwithin the mill isreflected at the exhauster by an increase suction dueto the reduced air flow and the dropping of part of the coal which wasin suspension back into the mill. At such times it is desirable to givethe mill an opportunity to clear itself before any additional coal isadded. From the above it will be seen that the suction at the exhausteris an indication of conditions occurring in the mill and we propose toutilize this suction to overcome difficulties herein fully pointed out.

For example, let us assume that the boiler is operating at a maximumrating and that the feeder, mill, and exhauster are running at theirmaximum speeds. Under this condition the feeder is supplying coalrapidly to the mill so that a considerable storage of coal takes placein the mill, which is desirable in order to prevent the mill frompounding at its high speed. Since the mill and exhauster, however, arebeing driven at their maximum speeds proper introduction of fuel intothe furnace takes place. Let us assume that the boiler rating is nowsuddenly reduced to the point where the feeder, mill and exhauster arebrought to their slowest speeds by the combustion control mechanism. Theair flow which is governed by the exhauster speed then drops oilpromptly and much of the fines which are in suspension drop back intothe mill. Since the mill speed is also greatly reduced the plows of themill will not throw up. the coal sufficiently to be picked up by the airfor delivery to the exhauster and burner. It will be seen, therefore,that under the above conditions coal stores up in the mill, and as thefeeder is still delivering coal to the mill, although at reduced speed,still more coal enters the mill to bring about an overload condition.

One of the primary objects of our invention is to overcome the abovedifliculties.

Under other conditions of operation there is a period in whichinsufficient coal is in the mill so that rumbling of the mill takesplace. For example, with a sudden increase in the load taken from theboiler, the mill, exhauster, and feeder willbe speeded up by thecombustion control 195 mechanism so that there would not be suflicientcoal in the mill to prevent a sudden decrease in the suction dropthrough the mill, and while the control mechanism would eventually speedthe feeder to the point where a proper level of coal This inventionrelates to controls for boiler installations and will be described inconnection with installations in which the furnace is directly firedfrom a mill such as a roller mill, i. e., installations in which afeeder receives coal from a bin and delivers it to a pulverizer millfrom which it is drawn by an exhauster and delivered into the furnace bymeans of suitable burners.

In such installations difliculties in firing are encountered becauseunder certain conditions the mill becomes overloaded with coal. Underother conditions there is not suflicient coal in the mill to ensureproper operation. The conditions referred to will be understood from thefollowing.

In steam generating installations of the character described it has beenfound that the mills tend to overload due either to a high percentage offines being delivered into the mill at any one time, changes in the typeof coal, or to a sudden reduction in boiler rating which finds the millwith more coal than is necessary for the reduced rating.

We are aware that automatic combustion controls have been employed insuch installations in which the speeds of the various pieces of fuelhandling equipment are changed to take care of the new load conditions,but we have found that there is a periodin which the mill is overloadedbecause of the storage of coal in it that took place when the boiler wasoperating at higher ratings and, therefore, even though the speeds ofthe devices be reduced at such a time, the overload condition will befurther aggravated rather than helped.

Stated more fully the known automatic combustion controls do not use thepositional idea. By this we mean that they do not take any deflniteposition for any particular steaming rate of the boiler. They simplyincrease or decrease the speeds of the motors of the various pieces offuel handling equipment whenever a change in pressure in the steamheader line occurs. Consequently when the steaming rate changes theslight change in header pressure causes the controls to be moved to anew position which will again give the present headerpressure. While themotion of the control may be practically instantaneous it does notnecessarily follow that the equipment can immediately adapt itself tothe new conditions. Such a'case occurs when the steam demand is reducedover a wide range in a very short interval of time. The exhauster,pulverizer and feeder motors will be brought down to their slower speedpositions, but, due

would exist in the mill, there nevertheless is a period in which thesupply would be inadequate to prevent rumbling.

We propose, therefore, to provide a system of control whereby suchrumbling is prevented.

We also contemplate the provision of a control system adapted to improvethe control of boiler installations under conditions when there is asubstantial drop of boiler rating from say 300% to 200% of rating.

Generally stated, therefore, it is an object of our invention to providesimple and effective control means for installations of the characterdescribed whereby proper coal conditions are ensured in the mill whenchanges in boiler rating take place.

A more specific object resides in the provision of eifective mill feedercontrol in boiler installations.

A further object resides in the provision of simple and effectivecontrol means in apparatus of the class described whereby approximatelyconstant or predetermined condition of suction may be maintained at theexhauster under different operating conditions.

How the foregoing, together with such other objects and advantages asmay hereinafter appear, or are incidentto our invention, are realized,is illustrated in preferred form in the accompanying drawings, wherein-Fig. 1 is an elevational view of a boiler installation embodying ourinvention; Fig. 2 is an enlarged elevational view of rheostat operatingmechanism which we employ; Fig. 3 is 'an end view of Fig. 2, and Fig. 4illustrates a simple form of wiring diagram.

The installation'illustrated comprises in gen eral a boiler A of anywell known type, a pulverized coal burning furnace B having suitableburner means 0 for the introduction of the fuel, a pulverlzer mill D, towhich coal is delivered by a feeder device E, a bin F for supplying coalto the feeder device, and an exhaust fan or exhauster G for drawingpulverized coal from the mill D for delivery into the furnace throughthe medium of the burner means C. A conduit 5 connects the mill to thesuction side of the exhauster and a conduit 6 connects the dischargeside of the exhauster to the burner C. Electric motors 7, 8 and 9 areemployed for driving the feeder E, the mill D and the exhauster Grespectively.

Control mechanism indicated as a whole by the reference letter H isprovided for automatically controlling the feeder, mill, and exhaustermotors. In general the control mechanism comprises three automaticallyoperated rheostats 10, 11 and 12, one for each motor, the rheostat 10being for the feeder motor 7, rheostat -11 for the mill motor 8 andrheostat 12 for the exhauster motor 9, and steam pressure actuatedmechanism J for causing the automatic operation of the rheostats.

A cam shaft 13 .(see Figs. 2 and 3) carrying three cams 14, 15 and 16 isprovided and the cams operate levers 17, 18 and 19 pivoted on a support20. The lever 17 is actuated by the cam 14 and its movement istransmitted to the feeder rheostat 10 by means of a flexible chain orcable 21 which is secured at one end to the free end 22 of the lever 17and carries a counterweight 23 at its other end. The flexible chain 21passes around a sprocket wheel 24 secured on-the rheostat shaft 25 andthen over an idler sprocket 26 from which the counterweight depends.Thus it will be seen that when the lever moves up and down the sprocketwheel 24 and rheostat 10 are correspondingly turned.

Inorder to initially set the rheostat or to correct its setting to apreselected position, we have provided a hand operated setting orcorrecting device 27 adapted to operate on the chain 21, which devicecomprises a member 28 secured on a rotatable shaft 29 having anoperating handle 30, and a pair of sprockets 31, 31, rotatably carriedby the member 28, between which the chain passes. When the handle 30 isrotated the run of chain between the lever 17 and sprocket wheel 24 islengthened or shortened according to the direction in which the member28 is rotated and this, in turn, rotates the sprocket 24 and rheostat 10either in a clockwise or counter-clockwise direction to afford settingof the rheostat as desired. The device 27 may be locked in its adjustedposition in any suitable manner.

Similar mechanisms indicated at 32 and 33 are employed for actuating themill and exhauster rheostats l1 and 12 respectively, through operationof the levers 18 and 19 associated with the cams 15 and 16. Handoperated setting or correcting devices 34 and 35 are also provided inthe mechanisms 32 and 33.

The cam shaft 13 is actuated by means of a Well known combustion controldevice indicated at J which is of the type receiving steam for itsoperation from the boiler. In general the device includes a pressureactuated piston 37 which is subjected to pressure under control of avalve device 38 operated by boiler steam pressure supplied thereto by aline 39 leading from a steam header 40 of the boiler. When the steampressure in the line 39 increases, the piston 37 moves upwardly and thismovement is transmitted to the cam shaft 13 by means of a link 41connected at one end to the piston rod 42 and at its other end to alever 43 secured to the cam shaft. The cam shaft is thus rotated in adirection to cause the cams 14, 15, and 16 to rock the levers 17, 18 and19 upwardly and this in turn operates the rheostats in a manner to slowdown the motors of the feeder, the mill, and the exhauster. When thesteam pressure drops the reverse operation takes place and the motorsare speeded up.

From the foregoing it will be seen that automatic control is providedwhereby the motors of the various pieces of equipment are slowed downwhen there is an increase in steam pressure and speeded up when there isa steam pressure drop.

Under conditions of gradual decreases and increases in steam pressurethe above system of control is quite satisfactory for controlling thefiring of the furnace to suit changes in load and boiler rating.However, there are conditions in many installations where the changesare sudden and rapid which are not effectively taken care of.

This is particularly true in a steel plant where changes in rating arevery rapid so that difficulties are encountered in firing due toconditions in the mill, as has been pointed out above.

In order to overcome such difficulties we supplement the above controldevice by suction operated devices adapted to additionally control themill feeder as will now appear.

A pipe connection 44 is taken off the exhauster inlet elbow 45 or atsome other convenient point in the mill where there is a maximum amountof suction, and is led to a well known type of suction relay device 46having a pivoted lever 47 adapted to be rocked through the action of thesuction on the relay. The pivoted lever 47 is connected, by means of alink 48, to any well known form of mercoid switch 49 or other type ofswitch adapted to make or break the feeder mo tor circuit. As will beclear from the wiring diagram illustrated in Fig. 4, .the mercoid switch49 is connected into the feeder motor circuit, and, therefore, serves asa controlling medium to start or stop the feeder motor. 7, the speed ofwhich is automatically regulated under normal conditions as hereinbeforedescribed.

Referring to the wiring .diagram the circuit for the exhauster motor .9.is shown with the connections such that the motor is operating. Anysuitable form of starting switch 50 may be employed. The rheostat 12. isshown in a position in which the motor field is weakened approximatelytwo-thirds.

The circuit for the pulverizer motor 8 is similar to that just describedand includes a suitable form of starting switch 51, therheostat 11 beingshown in position to weaken the motor field approximately two-thirds.

The circuit for the feeder motor 7 is shown with the connectionssuchthat the motor is operating with its field weakened approximatelytwo-thirds by the rheostat; 10. The automatic operation of the rheostats10, 11 and 12 has been described hereinbefore,

The feeder motor 'I is remotely controlled by a suitable start stopswitch 52 when the stop suction relay 46 is in the positioncorresponding to suction insufficient to cause it to open the controlcircuit through the medium of the mercoid switch 49. However, when thesuction in the relay 46 reaches a predetermined value, the suction relayoperates to rock the mercoid switch in a direction to open the mercurycontact at 53 so that the circuit to a magnetic switch 54 isinterrupted, thus causing the switch to, open. This opens the feedermotor control circuit-and stops the feeder motor until the mill hascleared itself and the suction at the inlet side of the exhauster hasbeen again restored to its normal value. It is to be noted that with theparts in the position shown in the wiring diagram, the operation of thefeeder motor 'I is similar to that of the motors 8 and 9 abovedescribed.

Assuming now that the boiler is operating at high rating with thefeeder,- mill and axhauster running at their maximumspeeds and that theboiler rating is suddenly: reduced to the point where the motorrheostats 10, 11 and i2 immediately slow the motors to their slowestspeeds.

then the suction relay 46 is operated on by high suction due to causesfully pointed out above and the pivoted lever 47 is moved in a directionto open the feeder motoncircuit, with the result that the feed to themill is interrupted for a time and for such a length of time as it takesfor the mill to be cleared of sufllcient coal to cause a reduction ofthe suction to. normal value. when this occurs the suction relayautomatically is reset and the feeder motor circuit is closed to againstart the feeder in operation which will then be under control of itsrheostat 10.

when there is a sudden increase in the load taken from the boiler, themill is speeded up suddenly under control of its-automatic rheostat 10,but we have found that there is not sufiicient coal in the mill at thisinstant to prevent a sudden decrease in the suction drop thru the mill,and, therefore, a period exists -when the mill would rumble extensivelyunless provision is made to get additional coal into the mill. That isto say, the

, rapid increase in air flow due to speeding up the exhauster motorthrough means of the combustion control mechanism J would tend to clearthe mill quickly, and although the feeder motor is also accelerated toits maximum speed position as represented by the position of therheostat 10, the suction will still be too low for a period of time sothat it is desirable to accelerate the feed beyond that which wouldordinarily be used for regulation. This situation is reflected at theexhauster inlet and, therefore, we propose to connect to the suctionpipe 44, a second suction relay device 55, similar to the suction relaydevice 46. The suction relay device 55 is adapted to operate a mercoidswitch 56 which, in turn, controls a fast magnetic relay 57. The fastrelay S'lzcuts in or cuts out an additional field resistance 58associated with the rheostat 10 as indicated in the wiring diagram. Thecontacts 59 of the magnetic relay 57 are normally closed and when thesuction to the suction relay 55 drops to a predetermined value, it isoperated to move the mercoid switch 56 to a position in which a mercurycontact is made at 60 and this, in turn, causes the magnetic relay 57 tobe energized, thus opening the contacts 59. When the contacts 59 areopened the extra resistance 58 is included in the field circuit and thefeeder motor speeds up to its maximum speed. As soon as the suctionbuilds up to the proper value, the fast suction relay 55 resets anddeenergizes the magnetic relay 57, permitting the combustion controlmechanism H and J to again act as the controlling medium in its usualfashion.

We also propose to improve the coal feed in installations of thecharacter described by taking the change of feed in two steps undercertain conditions, for example, when there is a drop in boiler ratingfrom, say 300% to 200% of rating, i. e., a change of rating which is notsevere enough to warrant completely interrupting the feed butsubstantial enough to warrant bringing the feed down to the minimumvalue for a short time. In

44, adapted to operate a mercoid switch 62. The L suction relay device61 is set at a value slightly below the stop relay device 46 andactuates the mercoid switch 62 which, in turn, actuates a 1 slowmagnetic relay 63 operating to reduce the motor speed to its minimumvalue. but without interrupting the motor circuit as will be clear fromthe wiring diagram.

The contacts 64 of the magnetic relay 63 are normally open and when thesuction to the suction relay 61 reaches the predetermined value at whichthe relay is set, it is operated to move the mercoid switch 62 to aposition in which a mercury contact is made at 65 and this, in turn,causes the magnetic relay 63 to be energized, thus closing the contacts64 and putting the feeder motor field directly across the lines, therebycausing the motor to run at its minimum speed, no matter what positionthe rheostat is at under control of the mechanism J and H. Whenconditions become normal the mechanism J and H i Similar conditions mayoccur when the coal feeds irregularly due to changing fineness of theraw coal or hang up of the coal on account of moisture or otherconditions. Such conditions would be corrected by the operation of thesuction relays in addition to the speed changes effected by thecombustion control mechanism referred to.

The following is an example as to the approximate values at which thesuction relays would be set if it is desired to hold a suction of 4.25"water gage at the exhaust inlet. The fast suction relay 55 would be setso that the contacts 59 would open at 4.15" suction and close at 4.00"suction. The slow suction relay 61 would be set so that the contacts 64would open at 4.35 suction and close at 4.50" suction. The stop suctionrelay 46 would be 'set so that the contacts 54 would open at 4.75"suction and close at 4.60" suction. With such settings approximateconstant suction would be maintained at the exhaust inlet.

While the diagram illustrates the use of a D. C. feeder motor withregulation of the feed by adjustment of the shunt field, it is to beunderstood that we contemplate connecting the suction relays in a mannerto control two motors or a two speed motor as would be the case in A. C.circuits. Also in some instances we may utilize a jumper connection" inplace of the extra resistance 58.

From the foregoing it will be seen that we have provided a very simple,and effective control system for boiler,installations which operates toautomatically control the feeder, mill, and exhauster speeds to ensureproper firing under normal changes in load conditions and also operatesto additionally automatically control the feeder speed to ensure propercoal conditions in the mill when there is a sudden change in load.

While we have illustrated our invention in connection with a directlyfired boiler installation, it is to be understood that the advantages tobe derived therefrom may also be obtained in indirectly firedinstallations. Also in some installations very satisfactory results maybe obtained by using the stop suction relay device 46 without thesuction relay'devices 55 and 61.

We claim:

1. In combination of a boiler and furnace and means for firing thefurnace, said firing means including a pulverizer, a fuel bin, a feederfor feeding fuel from the bin to the pulverizer, an exhaust fan forfeeding pulverized fuel from the pulverizer to the furnace, automaticmeans for simultaneously controlling the speed of the feeder, thepulverizer and the exhaust fan responsive to variations of the steampressure in the boiler, and automatic means independent of said firstmentioned means for further controlling the suction of the exhaust fan.7

2. A pulverized coal burning installation including a boiler, a furnace,a mill for pulverizing the coal, a feeder for delivering coal to themill, an exhauster for drawing pulverized coal from the mill fordelivery to the furnace, automatic means responsive to variations of thesteam pressure of the boiler for simultaneously controlling the speed ofthe mill, the feeder and the exhauster and automatic means independentof said first mentioned means for further controlling the speed of thefeeder responsive to variations of the suction of the exhauster.

3. In a pulverized coal burning installation, the combination of aboiler; a furnace; a coal pulverizer, a coal feeder, and an exhaust fan,each having an electric driving motor; a rheostat controlling the speedof each motor; automatic means responsive to variations of the steampressure speed of the feeder responsive to variations of the rrheostats, manually operating means for adjust ing said rheostats, andautomatic means responsive to variations of the suction of the exhaustfan for controlling the speed of the feeder motor independently of thesetting of its rheostat.

4. A pulverized coal burning installation including a boiler, a furnace,a mill for pulverizing the coal, a feeder for delivering coal to themill, an exhauster fan for drawing pulverized coal from the mill anddelivering it to the furnace, automatic means for simultaneouslycontrolling the speed of the mill, feeder, and fan responsive tovariations of the steam pressure of the boiler, and automatic meansindependent of said first mentioned means for further controlling thespeed of the feeder responsive to variations of the suction at thesuction side of the fan.

5. In a boiler installation, the furnace of which is fired by directdelivery of fuel thereinto from a mill, the combination of a motordriven mill, a motor driven feeder for delivering fuel to the mill, amotor driven exhauster for drawing fuel from the mill and delivering itinto the furnace, automatic means for simultaneously controlling thespeed of said motors including a speed con troller for each motor, steampressure actuated mechanism for operating said controllers, means fordelivering actuating steam to said mechanism from the boiler, and otherautomatic means inde pendent of the first means for further controllingthe speed of the feeder motor including means responsive to variationsof the suction of the exhauster for controlling the speed of the feedermotor beyond the control afforded by the first mentioned control means.

6. In a boiler installation, the furnace of which is fired by directdelivery of fuel thereinto from a mill, the combination of a motordriven mill, a motor driven feeder for delivering fuel to the mill, amotor driven exhauster for drawing fuel from the mill and delivering itinto the furnace, automatic means for simultaneously controlling thespeed of said motors including a rheostat controlling each motor, meansfor operating the rheostats, steam pressure. actuated mechanism foractuating said operating means, means for delivering actuating steam tosaid mechanism from the boiler, and'other automatic means independ entof said first mentioned means for further,controlling the speed of thefeeder motor including a switch for said feeder motor, a suction relayadapted to open and close said switch, and a suction line leadingfromthe suction side of the exhauster to said suction relay.

'7. A pulverized coal burning installation including a boiler, afurnace, a mill for pulverizing the coal, a feeder for delivering coalto the mill, an exhausterfor drawing pulverized coal from the mill fordelivery to the furnace, an electric driving motor for the mill,anelectric driving motor for the feeder, an electric driving motor forthe exhauster, a rheostat in the mill motor circuit for controlling thespeed of the mill motor, a rheostat in the feeder motor circuit forcontrolling the speed of the feeder motor, a rheostat in the exhaustermotor circuit for controlling the speed of the exhauster motor,automatic means responsive to variations of the steam pressure of theboiler for simultaneously controlling the speed of said mill, feeder,and exhauster motors through the medium of their rheostats, andautomatic means independent of said first means responsive to variationsof the suction of the exhauster "for further controlling the operationof the feeder I of the boiler for simultaneously operating saidmotorincluding a magnetic switch in the feeder ,150

mamas motor circuit adapted to stop the motor, and a. switch forcontrolling said magnetic switch to stop and start the feeder motor.

8. A pulverized coal burning installation including a boiler, a furnace,a mill for pulverizing the coal, a feeder for delivering coal to themill, an exhauster for drawing pulverized coal from the mill fordelivery to the furnace, an electric driving motor for the mill, anelectric driving motor for the feeder, an electric driving motor for theexhauster, a rheostat in the mill motor circuit for controlling thespeed of the mill motor, a rheostat in the feeder motor circuit forcontrolling the speed of the feeder motor, a rheostat in the exhaustermotor circuit for controlling the speed of the exhauster motor,automatic means responsive to variations of the steam pressure of theboiler for simultaneously controlling the speed of said mill, feeder,and exhauster motors through the medium of their rheostats, andautomatic means independent of said first mentioned automatic meansresponsive to variations of the suction of the exhauster for furthercontrolling the operation of the feeder motor including a magneticswitch in the feeder motor circuit adapted to stop the motor, a magneticswitch in the field circuit of the feeder motor adapted to control thespeed of the feeder motor by control of the field, a switch forcontrolling the first mentioned magnetic switch .to stop and start thefeeder motor, and a switch for controlling the second mentioned magneticswitch to vary the speed of the feeder motor.

9. A pulverized coal burning installation including a boiler, a furnace,a mill for pulverizing the coal, a feeder for delivering coal to themill, an exhauster for drawing pulverized coal from the mill fordelivery to the furnace, an electric driving motor for the mill, anelectric driving motor for the feeder, an electric driving motor for theexhauster, a rheostat in the mill motor circuit for controlling thespeed of the mill motor, a rheostat in the feeder motor circuit forcontrolling the speed of the feeder motor, a rheostat in the exhaustermotor circuit for controlling the speed of the exhauster motor,automatic means responsive to variations of the steam pressure of theboiler for simultaneously controlling the speed of said mill, feeder,and exhauster m0- tors through the medium of their rheostats, andautomatic means independent of said first mentioned automatic meansresponsive to variations of the suction of the exhauster for furthercontrolling the operation of the feeder motor including a magneticswitch in the field circuit of the feeder motor adapted to cause thefeeder motor to speed up by control of the field thereof, and

a switch for controlling said magnetic switch to speed up the feedermotor.

10. A pulverized coal burning installation including a boiler, afurnace, a mill for pulverizing the coal, a feeder for delivering coalto the mill, an exhauster for drawing pulverized coal from the mill fordelivery to the furnace, an electric driving motor for the mill, anelectric driving motor for the feeder, an electric driving motor for theexhauster, a rheostat in the mill motor circuit for controlling thespeed of the mill motor, a rheostat in the feeder motor circuit forcontrolling the speed of the feeder motor, a rheostat in the exhaustermotor circuit for controlling the speed of the exhauster motor,automatic means responsive to variations of the steam pressure of theboiler for simultaneously controlling the speed of said mill, feeder,and exhauster motors through the medium of their rheostats, andautomatic means independent of said first mentioned automatic meansresponsive to variations 7 of the suction of the exhauster for furthercontrolling the operation of the feeder motor including a magneticswitch in the field circuit of the feeder motor adapted to cause thefeeder motor to slow down by control of the field thereof, and a switchfor controlling said magnetic switch to slow down the feeder motor.

11. A pulverized coal burning installation including a boiler, afurnace, a mill for pulverizing the coal, a feeder for delivering coalto the mill, an exhauster for drawing pulverized coal from the mill fordelivery to the furnace, driving means for the mill, driving means forthe feeder, driving means for the exhauster, automatic means forsimultaneously controlling the speeds of the said driving meansresponsive to variations of the steam pressure of the boiler, anautomatic means independent of said first mentioned automatic means forfurther controlling the speed of the driving means for the feederincluding a plurality of devices responsive to variations of the suctionof the exhauster each adapted to be set to operate at predeterminedvalues of suction, means associated with one of said devices adapted tostop the feeder driving means when the suction reaches a predeterminedvalue, means associated with another of said devices adapted to reducethe speed of the feeder driving means when the suction reaches apredetermined value lower than that at which said first device operatesand means associated with another of said devices adapted to increasethe speed of the feeder driving means when the suction drops to a valuebelow that at which the other devices operate.

HARVEY C. MI'I'I'ENDORF. ALBERT C. FOSTER.

